The activity of a series of osmium complexes as catalysts for the hydrogenation of nitrile-butadiene copolymers (NBR) was studied under 3.42 MPa hydrogen pressures and at 130 degrees C in monochlorobenzene. On the basis of the correlation of phosphine ligand properties with the catalytic activities of complexes, the complexes studied are divided into three classes: class I (bulky monophosphines with Tolman's cone angle greater than or equal to 160 degrees) OsHCl(CO)(O-2)(PR3)(2) (PR3 = (PPr3)-Pr-1, PCy3, PCy2Ph); class LI (small monophosphines) OsHCl(CO)(PR3)(3) (PR3 = PPh3, P(m-C6H4Me)(3)); class III (diphosphines) OsHCl(CO)(PPh3)(dppp) (dppp = Ph2P(CH2)(3)PPh2). In general, the activity of these complexes decreases as follows: class I > class LI > class III. This trend is mainly attributed to the ease of dissociation of a ligand from an 18-electron complex to generate a 16-electron species (O-2 > PR3 > dppp) in the catalytic process. In the case of complexes of class I as catalysts, their activities increase in the order: PCy2Ph << (PPr3)-Pr-1 < PCy3. While this activity trend does not appear to correlate with the steric effect based on Tolman's cone angles, it is in good agreement with the electronic effect, which is evaluated based on the infrared nu(CO) values of these complexes. The catalytic activity of there complexes increases with the decrease of nu(CO) value, which is consistent with the increase of the donor power of phosphine ligands. This indicates that the activity of these complexes are predominately dependent on the electronic properties of phosphines and, in conjunction with the kinetic isotope effect, the rare determining step of the catalytic process would be an H-2 bond breaking related process or an olefin insertion into an Os-H bond. With complexes of classes II and III, a complex containing a bulky, strong a-donor and weak pi-acceptor phosphine is a good catalyst whereas that containing a chelating phosphine ligand would he a poor catalyst.